1. Field of the Invention
The present invention relates to an image forming device, and more particularly to an image forming device which prevents a banding phenomenon caused by uneven feeding of a recording medium.
2. Description of Related Art
FIG. 1 is a side view showing a printer unit 1a included in a color inkjet printer as a conventional image forming device. The inkjet printer performs printing based on obtained data on a recording sheet using the printer unit 1a. The printer unit 1a includes a feed roller 101, a pinch roller 102, a discharge roller 103, spurs 104 and 105, a line feed motor (not shown) for rotating the rollers 101 and 103, a sheet sensor 106 for detecting the presence or absence of a recording sheet, a platen 108, and an inkjet head 109. In FIG. 1, a recording sheet led into the printer unit 1a is fed in a sheet feeding direction indicated by an arrow Z.
The inkjet head 109 is mounted in a carriage 64 to be described later (see FIG. 3) so as to move back and forth in a direction perpendicular to the sheet feeding direction Z (perpendicularly to the plane of FIG. 1). Ink cartridges (not shown) each filled with one of the following colors of ink, namely cyan, magenta, yellow, and black, supply the inkjet head 109 with the ink. The inkjet-head 109 ejects the supplied ink from any one of the large number of nozzles formed on the inkjet head 109 onto the recording sheet, thereby printing an image. The inkjet head 109 has a plurality of nozzles for each color, at regular intervals in substantially parallel with the sheet feeding direction. The platen 108 is opposed to the inkjet head 109. The recording sheet is fed between the platen 108 and the inkjet head 109.
In the printer unit 1a, the feed roller 101 is provided upstream of the inkjet head 109 in the sheet feeding direction. The feed roller 101 and the pinch roller 102 facing the feed roller 101 sandwich the recording sheet therebetween, and then send the recording sheet forward by the rotation of the rollers. Passing through the feed roller 101, the recording sheet is fed immediately below the inkjet head 109.
The discharge roller 103 is disposed downstream of the feed roller 101 and the inkjet head 109. The discharge roller 103 and the spur 104 facing the discharge roller 103 also sandwich the recording sheet therebetween, and then send the recording sheet forward. The spur 104 is a rotational body with an uneven surface. The spur 104, disposed above the discharge roller 103, comes into contact with an ink image on a printed surface of the recording sheet. Since the ink image does not get dry immediately after printed, if the ink image contacts a roller having a large contact area before the image get dry, the printed image may be blurred, crinkled, or transferred to the roller, thereby degrading printing quality. Therefore, the spurs are employed in this conventional image forming device so as to reduce the contact area that contacts the printed surface of the recording sheet, thereby preventing printing quality from degrading.
The feed roller 101 and the discharge roller 103 are driven by the line feed motor (not shown) as the drive source for transporting a recording sheet. This allows the feed roller 101 and the discharge roller 103 to rotate in the sheet feeding direction. The rollers 101 and 103, together with the roller 102 and the spur 104 facing the rollers 101 and 103 respectively, sandwich the recording sheet therebetween, and then send the recording sheet forward. Note that the discharge roller 103 rotates faster than the feed roller 101, while the feed roller 101 sandwiches the recording sheet stronger than the discharge roller 103. Thus, the recording sheet disposed on the region opposing the inkjet head 109 is tautened, thereby preventing printing quality from degrading.
Therefore, while the recording sheet is being sandwiched by the feed roller 101 and the pinch roller 102, specifically until a trailing edge of the recording sheet passes through the nip between the feed roller 101 and the pinch roller 102, the feed roller 101 continues to send the recording sheet forward.
On the other hand, after the trailing edge of the recording sheet has passed through the nip between the feed roller 101 and the pinch roller 102, the recording sheet is fed by the rotation of the discharge roller 103.
While the recording sheet is fed by the feed roller 101 and the pinch roller 102 sandwiching the recording sheet, the sheet feeding accuracy is maintained. However, when the recording sheet reaches a point where the recording sheet leaves the feed roller 101 and the pinch roller 102, the trailing edge of the recording sheet is released from the pressure which has been applied on. This means that the sheet feeding accuracy becomes unstable due to the sheet being flipped, thereby causing a banding phenomenon.
Japanese Patent Application Laid-Open Publication No. 11-207945 and Japanese Patent Application Laid-Open Publication No. 11-291506 propose increasing a resolution level in the region where a banding phenomenon occurs so as to prevent the banding phenomenon. Japanese Patent Application Laid-Open Publication No. 2005-138501 discloses the method of decreasing the feeding amount of a recording medium to prevent the occurrence of a banding phenomenon.
However, although a banding phenomenon is prevented to some extent by these inventions, its banding pattern is still recognizable.
FIGS. 2A and 2B simulate how a banding phenomenon occurs. In these figures, each circle represents a dot formed with ink. FIG. 2A shows a case in which printing is performed while a recording sheet is fed by the feed roller 101 and the pinch roller 102 sandwiching the sheet. The dots are arranged systematically in a grid pattern. The expression “normal arrangement” in FIG. 2 means that a raster line is formed with dots aligned in a direction perpendicular (orthogonal) to the sheet feeding direction (Z direction in the figure), and one raster line is disposed on another raster line in sequence in the feeding direction.
FIG. 2B shows a case in which printing is performed in normal arrangement when a recording sheet is sent forward suddenly to an extreme degree. In this case, a linear space appears as indicated by two parallel solid lines in a direction perpendicular to the sheet feeding direction. This phenomenon causes a white line to be recognizable in a direction perpendicular to the sheet feeding direction.